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-==========
-PG Removal
-==========
-
-See OSD::_remove_pg, OSD::RemoveWQ
-
-There are two ways for a pg to be removed from an OSD:
-
-  1. MOSDPGRemove from the primary
-  2. OSD::advance_map finds that the pool has been removed
-
-In either case, our general strategy for removing the pg is to
-atomically set the metadata objects (pg->log_oid, pg->biginfo_oid) to
-backfill and asynronously remove the pg collections.  We do not do
-this inline because scanning the collections to remove the objects is
-an expensive operation.
-
-OSDService::deleting_pgs tracks all pgs in the process of being
-deleted.  Each DeletingState object in deleting_pgs lives while at
-least one reference to it remains.  Each item in RemoveWQ carries a
-reference to the DeletingState for the relevant pg such that
-deleting_pgs.lookup(pgid) will return a null ref only if there are no
-collections currently being deleted for that pg. 
-
-The DeletingState for a pg also carries information about the status
-of the current deletion and allows the deletion to be cancelled.
-The possible states are:
-
-  1. QUEUED: the PG is in the RemoveWQ
-  2. CLEARING_DIR: the PG's contents are being removed synchronously
-  3. DELETING_DIR: the PG's directories and metadata being queued for removal
-  4. DELETED_DIR: the final removal transaction has been queued
-  5. CANCELED: the deletion has been canceled
-
-In 1 and 2, the deletion can be canceled.  Each state transition
-method (and check_canceled) returns false if deletion has been
-canceled and true if the state transition was successful.  Similarly,
-try_stop_deletion() returns true if it succeeds in canceling the
-deletion.  Additionally, try_stop_deletion() in the event that it
-fails to stop the deletion will not return until the final removal
-transaction is queued.  This ensures that any operations queued after
-that point will be ordered after the pg deletion.
-
-OSD::_create_lock_pg must handle two cases:
-
-  1. Either there is no DeletingStateRef for the pg, or it failed to cancel
-  2. We succeeded in canceling the deletion.
-
-In case 1., we proceed as if there were no deletion occurring, except that
-we avoid writing to the PG until the deletion finishes.  In case 2., we
-proceed as in case 1., except that we first mark the PG as backfilling.
-
-Similarly, OSD::osr_registry ensures that the OpSequencers for those
-pgs can be reused for a new pg if created before the old one is fully
-removed, ensuring that operations on the new pg are sequenced properly
-with respect to operations on the old one.